MEMS pressure sensors, microphones, accelerometers, gyroscopes, switches, resonators and ultrasound transducers are examples that use movable membranes or beams.
A MEMS galvanic switch is one example of MEMS device which uses a movable element. A MEMS switch comprises a first electrode arrangement that is present on a substrate and a movable element that overlies at least partially the first electrode arrangement. The movable element is movable towards the substrate between a first and a second position by application of an actuation voltage, providing electrostatic attraction.
In the first position, the movable element is separated from the substrate by a gap. The movable element comprises a second electrode that faces the first electrode arrangement. In the second position (closed switch) first and second electrodes are in mechanical and electrical contact with each other.
Known MEMS switches of this type can use electrostatic actuation in which electrostatic forces resulting from actuation drive voltages cause the switch to close. An alternative type uses piezoelectric actuation, in which drive signals cause deformation of a piezoelectric beam.
Electrostatic galvanic MEMS switches are promising devices. They usually have 4 terminals: signal input, signal output, and two actuation terminals, one of which usually is kept at ground potential. By varying the voltage on the other actuation terminal, an electrostatic force is generated which pulls the movable structure downward. If this voltage is high enough, one or more contact bump electrodes will touch and will provide a galvanic connection between the two signal terminals.
FIGS. 1 and 2 show one possible design of MEMS galvanic switch designed in accordance with known design principles.
In FIG. 1, the cross hatched pattern is the bottom electrode layer. This defines the signal in electrode 10, the signal out electrode 12 and lower actuation electrode pads 14. As shown, the actuation electrode pads 14 are grounded.
A top electrode layer defines the movable contact element 16 as well as the second actuation electrode 18 to which a control signal (“DC act”) is applied.
The second actuation electrode 18 has a large area overlapping the ground actuation pads so that a large electrostatic force can be generated. However, because the top actuation electrode 18 and the movable contact element 16 are formed from the same layer, a space is provided around the movable contact element 16.
FIG. 2 shows the device in cross section taken through a vertical line in FIG. 1. The same components are given the same reference numbers. FIG. 2 additionally shows the substrate arrangement 2 and the gap 20 beneath the movable contact element 16.
The connection between the signal input and signal output electrodes is made by the movable contact electrode which has two contact bumps as shown in FIG. 2. Galvanic MEMS switches can achieve low resistances Ron of less then 0.5 Ohm when they are switched on, and high isolation with small parasitic capacitance when they are off (Coff<50 fF). Typical dimensions are 30 to 100 μm outer diameter of the actuation electrode 18.
The device is manufactured in well known manner, in which sacrificial etching defines the gap 20. Patterned etching of the sacrificial layer enables the contact bumps to be formed.
There is a general need to balance the requirement for low resistance contacts with the requirement to prevent sticking and arcing of the switch by ensuring that the actuating mechanism has sufficient restoring force to return the switch to its unactuated state once the contact force is removed.
A MEMS switch is one example of device where the movable beam deliberately makes contact with an underlying surface.
An example of a MEMS device in which contact is not made during normal operation is a MEMS microphone. FIG. 3 is an example of a MEMS condenser microphone. The sound pressure can be applied from the bottom or from the top.
The structure is formed on a silicon on insulator substrate 30, and comprises a top electrode 31 suspended by a spacer layer 32 over a movable electrode membrane 34.
The microphone is formed as an integrated MEMS device, in which the movable membrane 34 is suspended over an opening in the semiconductor substrate 30. The top electrode 31 functions as a back electrode and has perforations to allow the flow of air so that the membrane can move. The membrane is exposed to the sound pressure at an acoustic inlet beneath the substrate opening, and the microphone is enclosed in a package with sufficient volume of air to not hamper the membrane movement.
The capacitance between the two electrodes 31 and 34 defines the electrical microphone signal.
The microphone has an essentially linear response to the sound pressure when it is within limits. In the event of high sound pressure, the movable membrane makes contact with the perforated back electrode.
A well known failure mode observed in micro-mechanical systems (MEMS) is stiction of moveable elements. Stiction means that the moveable element is stuck to a fixed part of the device. The device loses its functionality.
This problem can arise especially in devices where contact between the movable element and the underlying substrate is not a normal part of the device operation. For example, MEMS microphones, accelerometers, resonators, variable capacitors or switches can all employ vertically movable elements, which do not necessarily need to make contact for normal operation of the device.
This stiction can be caused for example by surface tension of liquids during processing of the MEMS. During operation, adhesion, moisture effects, electrostatic forces and dielectric charging can lead to stiction as well. The stiction force increases when the area of contacting surfaces increases. Therefore, decreasing the area of contacting surfaces is an effective way of avoiding permanent stiction and failure of the MEMS device. This is one purpose of the contact bumps in FIG. 2.
Electrostatic forces (when a bias voltage is applied) and dielectric charging can keep the membrane in the stuck position after the membrane was brought into contact with the counter electrode. The thinner the dielectric between membrane and counter-electrode, the higher is the electrostatic force. Electrostatic MEMS devices therefore exhibit an electrostatic hysteresis and can only be released after contact by lowering the applied voltage. This effect can be reduced by a smaller contact area, but also by adding an insulator or increasing the insulator thickness between the (conducting) surfaces.
Manufacturing such bumps for addressing the stiction problem typically requires extra layers and extra mask steps during manufacturing.
The invention relates to designs and manufacturing methods for providing the desired low contact area and/or dielectric thickness in order to address the stiction problems explained above.